Advances in MOSFET technology as well as advances in integrated circuits have made it possible to apply class D amplifiers to audio applications. A typical Class D amplifier 600 is shown in FIG. 5. The control circuit 660 drives a level shifter 661, which in turn drives upper and lower power fets. The output of the fets is applied to a speaker through a filter, which converts the digital output of the fets into an audio analog signal.
Class D amplifiers are significantly more efficient than class AB amplifiers. The disadvantages are higher part count, cost, electromagnetic interference, and poor performance. With increased integration and the introduction of sophisticated control integrated circuits these disadvantages are becoming less pronounced. In the near future, class D amplifiers will replace class AB amplifiers in many applications. Class D amplifiers already have a clear advantage in high power applications. As the cost and component count of these amplifiers fall, class D amplifiers will be able to complete with class AB amplifiers in low and medium power applications.
To overcome the poor performance of class D amplifiers, others have suggested a self oscillating variable frequency modulator as shown in FIG. 6. An integrator 610 has an audio input over an input resistor R.sub.IN. It has a digital feedback input A over resistor R.sub.DFB, and an analog feedback at input B over resistor R.sub.AFB. The respective analog and digital feedback signals A, B, are taken from the output of the bridge circuit 620 and the low-pass filter that comprises the inductor L and capacitor C.sub.LP. For purposes of understanding, let us simply focus on the digital output A and assume that there is no audio input. In this case, the output at point A is a square wave with a 50% duty cycle. When the square wave is high, current flows through R.sub.DFB into the summing junction of the integrator 610. Its output ramps down until it reaches the negative threshold of the comparator 612. R1 and R2 are used to add hysteresis to the comparator 612. These resistors can be used to adjust the comparator positive and negative thresholds. When the output of the comparator 612 goes low, the upper FET 622 turns off and after a short delay the lower FET 624 turns on. The square wave goes low, and current now flows out of the integrator 610 summing junction through R.sub.DFB. The output of the integrator 610 reverses and ramps up until it reaches the positive threshold of the comparator 612. This signals the lower FET 24 to turn off. After a short delay the upper FET 622 turns on. The square wave goes high and the cycle continues. With no audio signal, the output at A is a 50% square wave, and the output of the integrator 610 is a triangle wave.
A typical prior art overcurrent scheme is shown in FIG. 7. A sense resistor, 631, is placed in the return path of the speaker. The voltage drop across this resistor is proportional to the speaker current. A low pass filter comprising inductor 632 and capacitor 633 removes switching noise. Two comparators 634, 635 compare the voltage across the sense resistor to two adjustable thresholds (+/- 100 mv in this example). If the voltage across the sense resistor exceeds 100 mv or is less than -100 mv, then the output of OR gate 36 will transition high indicating that an overcurrent event has been detected.
The prior art circuit has a number of drawbacks. It consumes output power and thereby reduces the maximum possible efficiency of the amplifier. The sense resistor can only protect the amplifier from short circuits across the speaker. Shorts from the positive speaker terminal to ground bypass the sense resistor. Thus, no short circuit protection exists in this condition. The prior art circuit is slow and requires a filter to remove the switching noise. The filter introduces undesirable delays. If this circuit is used to implement overcurrent latch off, these delays will likely not cause a problem. If, on the other hand, this detection circuit is used to implement active current limit, these delays will make the current loop difficult to stabilize. The sense resistor is expensive and, if it is undersized, it becomes a reliability risk.